The invention relates to a process for the production of the zinc salt of 2-mercaptopyridine-N-oxide in a form in which the product filters easily, washes easily and will form stable suspensions.
Under the name Zinkpyrion, the zinc salt of 2-mercaptopyridine-N-oxide is a well known commercial product. The compound has a broad spectrum activity against fungi and yeasts and its activity is independent of pH. The product is used in cosmetics, especially as and anti-dandruff agent in the manufacture of shampoo compositions and such compositions are described in copending U.S. patent application Ser. No. 360,555 filed Mar. 22, 1982, ABN which is incorporated herein by reference. In the industrial sector, suspensions of this zinc salt are used in those areas where protection from attack by bacteria, fungi and yeasts is desirable, e.g. for the storage preservation of dispersion paints and lacquers, acrylates or styrene-butadiene polymers, etc.
The preparation of the zinc salt of 2-mercaptopyridine N-oxide is known and it may be obtained by reaction of aqueous zinc salt solutions (e.g. zinc chloride, zinc sulfate) with aqueous solutions of the sodium salt of 2-mercaptopyridine-N-oxide at temperatures between 20.degree. and 80.degree. C., in particular at 50.degree. to 60.degree. C. The preparation of aqueous sodium-2-mercaptopyridine-N-oxide solutions has been described in the literature (U.S. Pat. No. 3,159,640). The zinc 2-mercaptopyridine-N-oxide thus prepared has an average grain size of 2 to 3 microns and consists mainly of cubic and octahedral crystals which are in part agglomerated. Such fine-crystalline precipitates are very difficult to isolate by filtration, which is very time-consuming and elution of extraneous ions (e.g. Na.sup.+ CI.sup.-) is laborious and also requires much time. Moreover, the resulting pastes still contain up to 45-48% water.
The product is marketed as an aqueous, white-creamy suspension and in an older application, copending, commonly assigned U.S. patent application Ser. No. 360,550 filed Apr. 6, 1982, ABN the suspension is obtained by the action of shearing forces of water-containing zinc salt of 2-mercaptopyridine-N-oxide in the presence of anionic surfactants (suspension with 48% active substance content) or non-ionic water-soluble cellulose derivatives (suspension with 40% active substance content). Despite the small crystal size, the stability of such suspensions is at most 2.5 to 3 months.
Because of these disadvantages, the urgent need and the problem to be solved was to prepare the zinc salt of 2-mercaptopyridine-N-oxide in a form easy to filter and yet can be incorporated into a stable suspension, or respectively to find a manufacturing process in which the zinc salt is obtained in this form.